1. Field of the Invention
The present invention relates to an electromagnetic device such as a stepping motor, a solenoid valve, or the like, used in an automotive continuously variable transmission, for example.
2. Description of the Related Art
FIG. 3 is an external view of a permanent-magnet stepping motor, FIG. 4 is a cross section taken along line IV—IV in FIG. 3, FIG. 5 is a cross section taken along line V—V in FIG. 4, FIG. 6 is a cross section taken along line VI—VI in FIG. 4, and FIG. 7 is a partial exploded perspective of the stepping motor in FIG. 4.
In the figures, a permanent-magnet (PM) stepping motor 1, which is immersed and used in an oil, includes: an outer casing 2 made of a resin; a tubular housing 12 made of a resin which is linked to the outer casing 2; a motor main body 3 disposed inside the outer casing 2; a shaft 4 functioning as a moveable shaft rotated by the motor main body 3; and a conversion mechanism 31 for converting rotation of the shaft 4 into rectilinear motion. Moreover, the outer casing 2 and the housing 12 constitute a cover.
The motor main body 3 includes a pair of stators 5 secured to the outer casing 2, and a rotor 6 secured to the shaft 4. The stators 5 have: coils 7 which are each constructed by winding a conducting wire in which an electrically-insulating layer is formed on a copper wire surface; coil terminals 8 led out from the coil 7; connector terminals 9 connected to the coil terminals 8; and an external connector 25 connected to the connector terminals 9. The rotor 6 has a bush 10 secured to the shaft 4, and a circumferentially-magnetized hollow cylindrical permanent magnet 11 fitted over and secured to the bush 10.
The housing 12 is fastened to the outer casing 2 by a plurality of screws 12A extending parallel to the shaft 4. A circular interfitting aperture 2a is formed in the outer casing 2, and an interfitting portion 12a for inserting into the interfitting aperture 2a is formed on the housing 12. As shown in FIG. 5, three positioning projections 12b, which protrude radially and come into contact with an inner circumferential surface of the interfitting aperture 2a, are formed on an outer circumferential surface of the interfitting portion 12a. Furthermore, an annular groove 12c is formed in a joining surface of the housing 12, where the housing 12 joins the outer casing 2.
A housing communicating aperture 12d communicating between internal and external portions of the housing 12 is disposed in a side surface portion of the housing 12. A filter 13 for catching contaminants contained in the oil is disposed in the housing communicating aperture 12d. The shaft 4 is rotatably held by a casing bearing 14 and a housing bearing 15. The housing bearing 15, which is secured inside the housing 12, is a rubber-seal type.
A rod 16 reciprocated in an axial direction of the shaft 4 by rotation of the shaft 4 is disposed at a tip portion of the housing 12. A base-end portion of the rod 16 is inserted inside the housing 12, and a tip portion of the rod 16 protrudes from the tip portion of the housing 12. A rod communicating aperture 16a communicating between the internal portion of the housing 12 and an internal portion of the rod 16 is formed in the rod 16. A sleeve 17 for guiding rectilinear motion of the rod 16, an oil seal 18 for preventing penetration of contaminants from an outer circumferential portion of the rod 16, and a ring-shaped stopper 19 for regulating progression of the rod 16 are each secured to an inner circumferential surface of the tip portion of the housing 12.
The conversion mechanism 31 includes a thread portion 4a, a guide member 20 made of a resin which is formed in the base-end portion of the rod 16 and is engaged with the thread portion 4a, and a stopper 21 made of a metal which is secured to the shaft 4 and regulates regression of the rod 16. Stopper surfaces 20b and 21a which are perpendicular to the direction of rotation of the shaft 4 are formed on the guide member 20 and the stopper 21, respectively. As shown in FIG. 6 a rotation-regulating projection portion 20a which protrudes radially and regulates rotation of the rod 16 is formed on an outer circumferential portion of the guide member 20. Consequently, the guide member 20 is displaced in an axial direction of the shaft 4 by rotation of the shaft 4. An operating member 22 made of a resin is mounted to the tip portion of the rod 16.
A construction of each of the stators 5 will now be explained in detail with reference to FIGS. 8 to 11.
As shown in FIG. 9, the coils 7 are each constructed by winding a conducting wire 50, shown in FIG. 8, formed by coating a copper wire 51 with an electrically-insulating layer 52 composed of a thermoplastic polyimide resin onto a bobbin 53 composed of nylon, which is a thermoplastic resin, for a predetermined number of winds. Then, end portions of the conducting wire 50 of each coil 7 are connected to the coil terminals 8 mounted to the bobbin 53. Furthermore, as shown in FIG. 10, the coil 7 wound onto the bobbin 53 is embedded in an outer molding 54 composed of nylon, which is a thermoplastic resin. In addition, as shown in FIG. 11, cores 55 made of iron are disposed so as to surround the coil 7, completing the construction of the stator 5.
The stepping motor 1 constructed in this manner is mounted to an automobile continuously variable transmission, for example, and the operating member 22 attached to the tip portion of the rod 16 is engaged with a link 40 which opens and closes a transmission control valve in the continuously variable transmission.
When an electric current is passed through the external connector 25, the coils 7 are magnetized, rotating the rotor 6 and the shaft 4 together. The guide member 20 is engaged in the thread portion 4a on the shaft 4, and since rotation of the guide member 20 is regulated, rotation of the shaft 4 is converted into rectilinear motion of the guide member 20 and the rod 16.
The transmission control valve is opened and closed through the link 40 by reciprocation of the rod 16, ultimately changing the rotational velocity ratio between the drive shaft and the engine shaft.
The conventional stepping motor 1 is mounted to an automobile continuously variable transmission, for example, and is entirely immersed in the oil, which contains sulfur and organosulfur compounds. The conducting wires 50 of the coils 7 are constructed by coating the copper wire 51 with the electrically-insulating layer 52, which is composed of the thermoplastic polyimide resin, through which the sulfur and organosulfur compounds permeate easily. For that reason, the sulfur and the organosulfur compounds in the oil permeate the electrically-insulating layer 52, reaching the copper wire 51. Furthermore, the coils 7 of the stator 5 are covered by the bobbin 53 and the outer molding 54, but because the bobbin 53 and the outer molding 54 are composed of the thermoplastic resin, through which the sulfur and organosulfur compounds permeate easily, the bobbin 53 and the outer molding 54 cannot block the sulfur and organosulfur compounds in the oil from reaching the electrically-insulating layer 52. As a result, chemical reactions occur at the surface of the copper wire 51 and organosulfur compounds are formed on the surface of the copper wire 51, giving rise to a state of decreased adhesive strength of the electrically-insulating layer 52 to the copper wire 51.
One problem has been that in this state, the electrically-insulating layer 52 may be breached due to interference between adjacent conducting wires 50 caused by repeated thermal expansion and thermal contraction due to the heat history of the conducting wires 50 themselves, leading to wire breakage or short circuiting between the conducting wires 50 caused by elution of copper due to electric potential differences between the conducting wires 50. Another problem has been that breaching of the electrically-insulating layer 52 of the conducting wires 50 is more likely at positions where the conducting wires 50 and the bobbins 53, which have different coefficients of thermal expansion, come into contact, leading to further short circuiting or wire breakage.
Yet another problem has been that when the temperature of the oil becomes greater than vaporization temperatures of volatile components in the oil due to heat generated by the coils 7, the electrically-insulating layer 52 of the conducting wires 50 is more likely to be permeated by sulfur, etc., and there is a greater likelihood of short circuiting occurring between the conducting wires 50.